Field of the Invention
The present invention relates to a method for manufacturing a thin film actuated mirror array in an optical projection system, and more particularly to a method for manufacturing a thin film actuated mirror array in an optical projection system for enhancing a light efficiency and for preventing damages of an active matrix, an actuator and a reflecting member.
In general, light modulators are divided into two groups according to their optics. One group is a direct light modulator such as a cathode ray tube (CRT), and the other group is a transmissive light modulator such as a liquid crystal display (LCD), a digital mirror device (DMD), and an actuated mirror array (AMA). The CRT projects a superior qualitative picture on a screen, but a weight, a volume, and a manufacturing cost of the CRT increase according to the magnification of the screen. The LCD has a simple optical structure, so a weight and a volume of the LCD are less than those of the CRT. However, the LCD has a poor light efficiency of under 1 to 2% due to light polarization. And, there are another problems in a liquid crystal material of the LCD such as sluggish response and overheating. Thus, the DMD and the AMA have been developed to solve these problems. Now, the DMD has a light efficiency of about 5% while the AMA has a light efficiency of above 10%. The AMA enhances a contrast of a picture projected on a screen, so the picture is more apparent and brighter. The AMA is not affected by the polarization of rays of incident light. Also, the AMA is not affected by the polarization of a reflected light. Therefore, the AMA is more efficient than the LCD or the DMD. FIG. 1 shows a schematic diagram for showing an engine system of a conventional AMA disclosed in U.S. Pat. No. 5,126,836 (issued to Gregory Um).
Referring to FIG. 1, a incident light from a light source 1 passes through a first slit 3 and a first lens 5 and is divided into red, green and blue lights according to the Red.Green.Blue (R.G.B) system of color representation. After the divided red, green, and blue lights are respectively reflected by a first mirror 7, and second mirror 9, and a third mirror 11, the reflected lights are respectively incident on AMA devices 13, 15, and 17 corresponding to the mirror 7, 9, and 11. The AMA devices 13, 15, and 17 respectively tilt the mirrors installed therein, so the rays of incident light are reflected by the mirrors. In this case, the mirrors installed in the AMA devices 13, 15, and 17 are tilted according to the deformation of active layers formed under the mirrors. The rays of reflected light by the AMA devices 13, 15, and 17 pass a second lens 19 and a second slit 21 and form a picture on a screen (not shown) by means of a projection lens 23.
In most case, zinc oxide (ZnO) is used as a material forming the active layer. However, lead zirconate titanate (PZT: Pb(Zr, Ti)0.sub.3) has been found to have a better piezoelectric property than ZnO. PZT is a complete solid solution made of lead zirconate (PbZrO.sub.3) and lead titanate (PbTiO.sub.3). At a high temperature, PZT exists in a paraelectric phase whose crystal structure is a cubic. While at a room temperature, PZT exists in an antiferroelectric phase whose crystal structure is an orthorhombic, in a ferroelectric phase whose crystal structure is a rhombohedral, or in a ferroelectric phase whose crystal structure is a tetragonal according to the composition ratio of Zr to Ti.
The PZT has a morphotropic phase boundary (MPB) of the tetragonal phase and the rhombohedral phase where the composition ratio of Zr to Ti is 1:1. PZT has a maximum dielectric property and piezoelectric property at the MPB. The MPB does not lie in a specific composition ratio but lies over a relatively wide region where the tetragonal phase and the rhobohedral phase coexist. The phase coexistent region of PZT is reported differently depending on researchers. Various theories such as thermodynamic stability, compositional fluctuation, and internal stress have been suggested as the reason for the phase coexistent region. Nowadays, a PZT thin film can be manufactured by various processes such as a spin coating method, a chemical vapor deposition (CVD) method, or a sputtering method.
The AMA is generally divided into a bulk type AMA and a thin film type AMA. The bulk type AMA is disclosed in U.S. Pat. No. 5,469,302 (issued to Dae-Young Lim). The bulk type AMA is formed as follows. A ceramic wafer having a multilayer ceramic where metal electrodes are inserted is mounted on an active matrix having transistors. After sawing the ceramic wafer, a mirror is mounted on the ceramic wafer. However, the bulk type AMA has some disadvantages that very accurate process and design are required, and the response of an active layer is slow. Therefore, the thin film AMA that can be manufactured by using semiconductor manufacturing technology has been developed.
The thin film AMA is disclosed at U.S. Ser. No. 08/814,019 entitled "THIN FILM ACTUATED MIRROR ARRAY IN AN OPTICAL PROJECTION SYSTEM AND METHOD FOR MANUFACTURING THE SAME, which is now pending in USPTO and is subject to an obligation to the assignee of this application.
FIG. 2 is a plan view for showing a thin film actuated mirror array in an optical projection system disclosed in a prior application of the assignee of this application, FIG. 3 is a perspective view for showing the thin film actuated mirror array in an optical projection system in FIG. 2, and FIG. 4 is a cross-sectional view taken along the line A.sub.1 -A.sub.2 of FIG. 3.
Referring to FIGS. 2 to 4, the thin film AMA has a substrate 50, an actuator 65 formed on the substrate 50 and a reflecting member 71 formed on the actuator 65.
The substrate 50 has an electrical wiring (not shown) for receiving a first signal from outside and transmitting the first signal, a connecting terminal 51 formed on the electrical wiring and connected to the electrical wiring, a passivation layer 52 formed on the electrical wiring and the connecting terminal 51, and an etching stop layer 53 formed on the passivation layer 51. The etching stop layer 53 protects the substrate 50 and the passivation layer 52 during successive etching process. Preferably, the electrical wiring has a metal oxide semiconductor (MOS) transistor (not shown) for switching operation.
The actuator 65 has a supporting layer 57, a bottom electrode 59 formed on the central portion of the supporting layer 57, an active layer 61 formed on the bottom electrode 59, a top electrode 63 formed on the active layer 61, a common line 67 formed on a portion of the supporting layer 57 and connected to the top electrode 63, and a post 70 formed on a portion of the top electrode 63. The supporting layer 57 has a first portion attached to the etching stop layer 53 having the connecting terminal 51 formed thereunder and a second portion formed parallel to the etching stop layer 53. An air gap 55 is interposed between the supporting layer 57 and the etching stop layer 53.
Referring to FIG. 4, the actuator 65 has a via contact 73 formed in the inside of a via hole 72 which is formed perpendicularly to the connecting terminal 51 from a portion of the supporting layer 57 having the connecting terminal 51 formed thereunder and a bottom electrode connecting member 75 formed from the via contact 73 to the bottom electrode 59. A first signal, a picture signal, is applied from outside to the bottom electrode 59 through the electrical wiring, the connecting terminal 51, the via contact 73, and the bottom electrode connecting member 75. At the same time, a second signal, a bias signal, is applied to the top electrode 63 from outside through the common line 67 so that the active layer 61 formed between the top electrode 63 and the bottom electrode 59 is deformed. Preferably, the supporting layer 57 has a T shape and the bottom electrode 59 having a rectangular shape is formed on the center portion of the supporting layer 57. The active layer 61 has a rectangular shape smaller than the bottom electrode 59 and the top electrode 63 also has a rectangular shape smaller than the active layer 61.
The reflecting member 71 for reflecting incident light is supported by the post 70 and is formed parallel above the top electrode 63. Preferably, the reflecting member 71 has a rectangular shape.
A method for manufacturing the thin film AMA disclosed in a prior application will be described below.
FIGS. 5A to 5D illustrate the manufacturing steps of the thin film AMA in an optical projection system illustrated in FIG. 4.
Referring to FIG. SA, the substrate 50 having an electrical wiring (not shown) for receiving the first signal from outside and transmitting the first signal to the bottom electrode 59 and the connecting terminal 51 is provided. Preferably, the substrate 50 is composed of a semiconductor such as silicon (Si) and the electrical wiring has the MOS transistor (not shown) for switching operation.
The passivation layer 52 is formed on the substrate 50 and the connecting terminal 51. The passivation layer 52 is formed by chemical vapor deposition (CVD) method so that the passivation layer 52 has a thickness of between 0.1 .mu.m and 1.0 .mu.m. The passivation layer 52 is formed by using a phosphor silicate glass (PSG) and protects the substrate 50 having the electrical wiring and the connecting terminal 51 during successive manufacturing process.
An etching stop layer 53 is formed on the passivation layer 52. The etching stop layer 53 is formed by using nitride so that the etching stop layer 53 has a thickness of between 1000 .ANG. and 2000 .ANG.. The etching stop layer 53 is formed by low pressure chemical vapor deposition (LPCVD) method. The etching stop layer 53 protects the substrate 50 and the passivation layer 52 during the successive etching process.
A first sacrificial layer 54 is formed on the etching stop layer 53. The first sacrificial layer 54 enables the actuator 65 composed of the film layers to be easily formed. The first sacrificial layer 54 will be removed by using hydrogen fluoride (HF) vapor when the actuator 65 is completely formed. The first sacrificial layer 54 is formed by using PSG so that the first sacrificial layer 54 has a thickness of 0.5 .mu.m and 2.0 .mu.m. The first sacrificial layer 54 is formed by using atmospheric pressure CVD (APCVD) method. In this case, the degree of flatness of the first sacrificial layer 54 is poor because the first sacrificial layer 54 covers the surface of the substrate 50 having the electrical wiring and the connecting terminal 51. Therefore, the surface of the first sacrificial layer 54 is planarized by spin on glass (SOG) method or chemical mechanical polishing (CMP) method. Preferably, the surface of the first sacrificial layer 54 is planarized by the CMP method.
Then, the first sacrificial layer 54 is patterned so as to expose the portion of the etching stop layer 53 having the connecting terminal 51 formed thereunder. A first layer 56 is formed on the first sacrificial layer 54 and the exposed portion of the etching stop layer 53. The first layer 56 is formed by using a material such as nitride so that the first layer 56 has a thickness of between 0.1 .mu.m and 1.0 .mu.m. The first layer 56 is formed by LPCVD method. The first layer 56 will be patterned to form the supporting layer 57.
Referring to FIG. 5B, a first photo resist 58 is coated on the first layer 56 by spin coating method. Then, the first photo resist 58 is patterned to expose the central portion of the first layer 56 perpendicular to the exposed portion of the etching stop layer 53.
A bottom electrode layer is formed on the exposed portion of the first layer 56 and the first photo resist 58 by sputtering method. Then, the bottom electrode layer is patterned with respect to a position at which the common line 67 will be formed so that the bottom electrode 59 having the rectangular shape is formed on the central portion of the first layer 56. The bottom electrode 59 is formed by using a metal having conductivity such as platinum (Pt), tantalum (Ta) or platinum-tantalum (Pt--Ta) so that the bottom electrode 59 has a thickness of between 0.1 .mu.m and 1.0 .mu.m.
A second layer 60 is formed on the bottom electrode 59 and the first photo resist 58. The second layer 60 is formed by sol-gel method, sputtering method or CVD method so that the second layer 60 has a thickness of between 0.1 .mu.m and 1.0 .mu.m, preferably 0.4 .mu.m. The second layer 60 is formed by using piezoelectric material such as barium titanic oxide (BaTiO.sub.3), PZT (Pb(Zr, Ti)O.sub.3) or PLZT ((Pb, La)(Zr, Ti)O.sub.3) or an electrostrictive material such as PMN (Pb(Mg, Nb)O.sub.3). Successively, the second layer 60 is annealed by rapid thermal annealing (RTA) method. The second layer 60 will be patterned so as to form the active layer 61.
A top electrode layer 62 is formed on the second layer 61. The top electrode layer 62 is formed by using a material having conductivity such as aluminum (Al), platinum (Pt) or tantalum (Ta). The top electrode layer 62 is formed by sputtering method so that the top electrode layer 62 has a thickness of between 0.1 .mu.m and 1.0 .mu.m. The top electrode layer 62 will be patterned so as to form the top electrode 63.
Referring to FIG. 5C, after a second photo resist (not shown) is coated on the top electrode layer 62 by spin coating method, the top electrode layer 62 is patterned by using the second photo resist as an etching mask so that the top electrode 63 having the rectangular shape is formed. The second layer 60 is patterned by using the same method as that of patterning the top electrode layer 62 so that the active layer 61 is formed. That is, a third photo resist (not shown) is coated on the top electrode 63 and the second layer 60 by spin coating method after the second photo resist is removed by etching. The second layer 60 is patterned by using the third photo resist as an etching mask so that the active layer 61 having the rectangular shape wilder than that of the top electrode 63 is formed. At this time, the active layer 61 has the rectangular shape smaller than that of the bottom electrode 59 formed previously.
The first layer 56 is patterned by using the above-described method so as to form the supporting layer 57. The supporting layer 57 has the T shape. The bottom electrode 59 is formed on the central portion of the supporting layer 57. After the first photo resist 58 is removed, the common line 67 is formed on a portion of the supporting layer 57. Namely, after a fourth photo resist (not shown) is coated on the supporting layer 57 by spin coating method, the fourth photo resist is patterned to expose the portion of the supporting layer 57. Then, the common line 67 is formed by using a platinum (Pt), a tantalum (Ta), a platinum-tantalum (Pt--Ta), an aluminum (Al) or silver (Ag). The common line 67 is formed by using sputtering method or CVD method so that the common line 67 has a thickness of between 0.5 .mu.m and 2.0 .mu.m. In this case, the common line 67 is separated from the bottom electrode 59 by a predetermined distance and a portion of the common line 67 is connected to the top electrode 63.
A first portion of the supporting layer 57 having the connecting terminal 51 thereunder and a second portion of the supporting layer 57 which is adjacent to the first portion of the supporting layer 57 are simultaneously exposed when the fourth photo resist is patterned. Subsequently, the via hole 72 is formed by etching from the first portion of the supporting layer 57 to the connecting terminal 51 through the etching stop layer 53 and the passivation layer 52. The via contact 73 is formed in the inside of the via hole 72 from the connecting terminal 51 to supporting layer 57. At the same time, the bottom electrode connecting member 75 is formed for connecting the via contact 73 to the bottom electrode 29. Therefore, the via contact 73, the bottom electrode connecting member 75 and the bottom electrode 59 are connected one after another. The via contact 73 and the bottom electrode connecting member 75 are formed by using a material having conductivity such as a platinum (Pt), a tantalum (Ta) or a platinum-tantalum (Pt--Ta) by using sputtering method or CVD method. In this case, the bottom electrode connecting member 75 has a thickness of between 0.5 .mu.m and 1.0 .mu.m. The fourth photo resist is removed by etching, so that an actuator 65 having the top electrode 63, the active layer 61, the bottom electrode 59 and the supporting layer 57 is completed.
Referring to FIG. 5D, a second sacrificial layer 68 is formed on the actuator 65 by using a material having fluidity such as a polymer. The second sacrificial layer 68 is formed by using spin coating method so that the second sacrificial layer 68 completely covers the top electrode 63. Successively, the second sacrificial layer 68 is patterned so as to expose a portion of the top electrode 63. The post 70 and the reflecting member 71 is respectively formed on the exposed portion of the top electrode 63 and on the second sacrificial layer 68 by using a material having reflectivity such as an aluminum (Al), a platinum (Pt) or a silver (Ag). The post 70 and the reflecting member 71 is formed by sputtering method or CVD method. Preferably, the reflecting member 71 which reflects an incident light from a light source (not shown) is a mirror and has a thickness of between 0.1 .mu.m and 1.0 .mu.m. Consequently, after the first sacrificial layer 54 and the second sacrificial layer 68 are removed by using hydrogen fluoride (HF) vapor, the actuator 65 and the reflecting member 71 are completed as shown in FIG. 3.
In the above-described thin film AMA, the second signal is applied to the top electrode 63 via the common line 67. At the same time, the first signal is applied to the bottom electrode 59 via the electrical wiring formed on the substrate 50, the connecting terminal 51, the via contact 73 and the bottom electrode connecting member 75. Thus, an electric field is generated between the top electrode 63 and the bottom electrode 59. The active layer 61 formed between the top electrode 63 and the bottom electrode 59 is deformed by the electric field.
The active layer 61 is deformed in a direction perpendicular to the electric field. Hence, the actuator 65 having the active layer 61 is actuated upward by a predetermined tilting angle. The reflecting member 71 tilts by the same tilting angle of the actuator 65 because the reflecting member 71 is formed on the actuator 65 and is supported by the post 70. Therefore, the reflecting member 71 reflects the incident light by a predetermined angle, so the picture is projected onto a screen through a slit.
However, in the above-described thin film AMA, it is difficult that the second sacrificial layer composed of a polymer has an even surface. Thus, the surface of the reflecting member is irregular and the flatness of the reflecting member is poor because the reflecting member is formed on the second sacrificial layer which has the uneven surface so that the light efficiency may be decreased. In addition, during removing the first sacrificial layer and the second sacrificial layer by using the hydrogen fluoride vapor, the substrate, the active layer and the reflecting layer are damaged by the hydrogen fluoride vapor.